As far as using aluminum for one-time (or infrequent) jigs/fixtures, sure. Why not?
There are a couple of drawbacks to using the aluminum. It's less stiff than cast iron or steel, so it won't be as rigid for an equivalent cross section. Assuming it's "just regular aluminum" (6061 or whatever) it won't wear as well as cast iron or steel. It's also softer, so it will dent more easily (assuming it's "just regular aluminum"). If you can live with those constraints...have at it. Even if the experiment is a dismal failure, you'll learn something by it.
I'm not sure I'd make a sliding taper attachment from aluminum; given the amount of work it would be to make it, I'd probably opt for a material that would stand up better, but I don't see why aluminum wouldn't work for a while at least...assuming you take the constraints of the material into consideration in your design.
Now, you can get some alloys of aluminum that are pretty hard and are reasonably good bearing surfaces, so with the correct alloy about the only drawback would be a lower rigidity. But odds are that's not what you have....
Try to make a living, not a killing. -- Utah Phillips
Don't believe everything you know. -- Bumper sticker
Everybody is ignorant, only on different subjects. -- Will Rogers
Law of Logical Argument - Anything is possible if you don't know what you are talking about.